Black stem rust is a fungal pest that attacks and kills wheat and barley plants. Rusts of various sorts have adapted to most types of grains cultivated by humans, and there are many types of rust that will attack wheat and barley. But black stem rust has been in the news lately because a particularly virulent strain (Ug99) has emerged in Africa and is threatening the world's grain supplies. Black stem rust is also one of the most common and deadly types of rust to attack wheat; because of this the terms "wheat rust" or "stem rust" generally refer specifically to black stem rust.


Kingdom: Fungi
Phylum: Basidiomycota
Class: Urediniomycetes
Subclass: Incertae sedis (the Latin way to say "we don't know where to put this".)
Order: Uredinales (the rusts)
Family: Pucciniaceae
Genus: Puccinia
Species: P. graminis
Forma specialis: Puccinia graminis tritici

The life cycle of stem rust alternates between cereal crops and barberry plants. The rust that we see on wheat and other cereal plants is technically called uredia (sing. 'uredinium'). It is the asexual stage of the rust, and grows parasitically on the host, often killing it. This rust is indeed rust colored. This rust consists of hundreds of tiny spots, each one a fully grown uredinium that has pushed its way through the stem wall and is busy producing spores. These urediniospores will reinfect other plants, forming new uredia, spreading thru the fields during the summer months.

As the summer comes to an end, the uredia stop producing urediniospores and instead start producing teliospores. These spores stay attached to the plant, and bed down for the winter. When they emerge in the spring, they are a new type of spore, a basidiospore. This spore cannot infect cereal crops, and instead they must find a barberry plant.

Barberries (Genus Berberis) are edible, and are harvested in some parts of the world to flavor meat dishes or candy. But you're probably most familiar with them as a landscaping shrub. Barberries can grow quite large, but they are usually used as a low groundcovering shrub. They often have bright berries, and can be found in many different colors. They also have sharp spines, making them a good 'stay off my garden' plant. They are an absolutely necessary part of the stem rust life cycle. The basidiospores infect the young leaves of a barberry plant, where it forms a discolored spot on the top of the leaves known as a pycnia.

The basidiospores bloom into spermogonia, which are the 'sexual organs' of the rust. They ooze out spermatia, the fungal equivalent of sperm and eggs. The spermatia are accompanied by a sugary nectar that attracts insects; rust is fertilized in the same manner as are flowers, as insects unwittingly carry spermatia from plant to plant. A fertilized basidiospore is known as an aecium, and will shortly start to produce aeciospores, which will be dispersed by the wind to fall on unsuspecting wheat and barley plants. Upon infecting these plants the aeciospores will form uredia, and the cycle will begin again.


In the early 1900s stem rust epidemics devastated American wheat crops. In response, the U.S. Department of Agriculture initiated a barberry eradication program in 1918. The program is credited with destroying millions of individual barberry plants on tens of thousands of farms. While stem rust was not eradicated, it was brought under control in the United States.

There was another American outbreak of stem rust in the 1950s, leading to renewed eradication attempts. Luckily, Norman Borlaug, working for the Mexican government and the Rockefeller Foundation, had recently started work on breeding rust resistant strains of wheat. These new strains became the preferred method of controlling stem rust. Borlaug spent 20 years genetically tinkering with wheat, breeding strains that were more resistant to disease and gave a higher yield. His efforts have helped feed millions of people, and helped make stem rust a comparatively minor crop pest.


In 1999 a new strain of black stem rust appeared in Uganda (hence the appellation Ug99). Ug99 is not deterred at all by the genetically modified strains of wheat, and it is estimated that about 80 percent of wheat currently grown in Asia and Africa is susceptible to infection. At the moment Ug99 has spread though East Africa and is working its way into the Middle East.

In 2007 a variant of Ug99 spread to Yemen, and was found to have adapted itself to be even more damaging to crops. If Ug99 continues to adapt and spread across the globe it will cause widespread starvation (since wheat provides about 20 percent of the world's food calories, this could mean 1,000,000,000 less people eating food -- or more likely, 5,900,000,000 people eating less food.) This is also coming at a time when many countries are running out of water for their crops, and when America, the world's greatest exporter of grain, is turning to biofules made from food crops. As food prices rise, poverty and death become more and more prevalent.

Attempts are being made to control the spread of Ug99, and scientists are trying to breed more resistant strains of wheat. At this point Ug99 is winning, but we've only just started fighting back. We will probably curb the spread of Ug99 before it effects the greater part of the globe, and in the meantime starvation in Africa has never had much political clout in developed countries. America may soon be cutting both food aid and funding for agricultural research (Go Bush!), but the nations more immediately affected are proactively banding together to fight the new strain of stem rust.

In the meantime, Ug99 stands as a reminder that the devil of genetic modification isn't all bad -- it saves millions of lives -- and that the dominant worldwide paradigm of monoculture is prone to sudden, devastating catastrophe. And, of course, as a reminder to kill barberries.


An Earth2 node.

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